Seminar Heat Pipe

7/27/2019 Seminar Heat Pipe

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CONTENTS_

Introduction

Basic components of a heat pipe

Container

Working fluid

Construction

Operating principleMechanism

Experimental Procedure

Experiment

Study of various parameters

Limitations

Conclusion


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HEAT PIPES

INTRODUCTIONA heat pipe is a device that efficiently transports thermal energy from

its one point to the other. It utilizes the latent heat of the vaporized

working fluid instead of the sensible heat. As a result, the effective

thermal conductivity may be several orders of magnitudes higher than

that of the good solid conductors. A

heat pipe consists of a sealed container, a wick structure, a small

amount of working fluid that is just sufficient to saturate the wick and

it is in equilibrium with its own vapor. The operating pressure inside

the heat pipe is the vapor pressure of its working fluid. The length of

the heat pipe can be divided into three parts viz. evaporator section,

adiabatic section and condenser section. In a standard heat pipe, the

inside of the container is lined with a wicking material. Space for the

vapor travel is provided inside the container.

Basic components of a heat pipe

The basic components of a heat pipe are

1.The container2.The working fluid


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3.The wick or capillary structure

Container

The function of the container is to isolate the working fluid from the

outside environment. It has to be there for leak proof, maintain the

pressure differential across the walls, and enable transfer of thermal

energy to take place from and into the working fluid.

The prime requirements are:


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1.Compatibility (Both with working fluid and Externalenvironment)

2.Porosity3.Wettability4.Ease of fabrication including welding, machinability and

ductility

5.Thermal conductivity6.Strength to weight ratio

Working fluid

The first consideration in the identification of the working fluid is the

operating vapor temperature range. Within the approximate

temperature band, several possible working fluids may exist and a

variety of characteristics must be examined in order to determine the

most acceptable of these fluids for the application considered.

The prime requirements are:

7.Compatibility with wick and wall materials8.Good thermal stability9.Wettability of wick and wall materials10. High latent heat11. High thermal conductivity12. Low liquid and vapor viscosities


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13. High surface tension

Wick

The wick structure in a heat pipe facilitates liquid return from the

evaporator from the condenser. The main purposes of wick are to

generate the capillary pressure, and to distribute the liquid around the

evaporator section of heat pipe. The commonly used wick structure is

a wrapped screen wick.

ConstructionA typical heat pipe consists of a sealed hollow tube, which is made

from a thermoconductive metal such as copper or aluminium. The

pipe contains a relatively small quantity of "working fluid" (such aswater, ethanol or mercury) with the remainder of the pipe being filled

with vapor phase of the working fluid.

On the internal side of the tube's side-walls a wick structure exerts a

capillary force on the liquid phase of the working fluid. This is

typically a sintered metal powder (sintering is a method for making

objects from powder, by heating the material until its particles adhere

to each other) or a series of grooves etched in the tube's inner surface.

The basic idea of the wick is to soak up the coolant.

Axial Groove Fine Fiber Screen Mesh Sintering


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Heat pipes contain no moving parts and require no maintenance and

are completely noiseless. In theory, it is possible that gasses may

diffuse through the pipe's walls over time, thus reducing this

effeciveness.

The vast majority of heat pipes uses either ammonia or water as

working fluid. Extreme applications may call for different materials,

such as liquid helium (for low temperature applications) or mercury

(for extreme high temperature applications).

The advantage of heat pipes is their great efficiency in transferring

heat. They are actually a better heat conductor than an mass of solid

copper.

Wicking

MaterialConductivity

Overcome

Gravity

Thermal

ResistanceStability

Conductivity

Lost

Axial Groove Good Poor Low Good Average

Screen Mesh Average Average Average Average Low

Fine Fiber Poor Good High Poor Average

Sintering r) Average Excellent High Average High

Operating principle

Figure shows the working principle of a heat pipe. Thermal input at

the evaporator region vaporizes the working fluid and this vapor


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travels to the condenser section through the inner core of heat pipe. At

the condenser region, the vapor of the working fluid condenses and

the latent heat is rejected via condensation. The condensate returns to

the evaporator by means of capillary action in the wick.

As previously mentioned there is liquid vapor equilibrium inside the

heat pipe. When thermal energy is supplied to the evaporator, this

equilibrium breaks down as the working fluid evaporates. The

generated vapor is at a higher pressure than the section through the

vapor space provided. Vapor condenses giving away its latent heat of

vaporization to the heat sink. The capillary pressure created in the

menisci of the wick, pumps the condensed fluid back to the

evaporator section. The cycle repeats and the thermal energy is

continuously transported from the evaporator to condenser in the form

of latent heat of vaporization. When the thermal energy is applied to

the evaporator, the liquid recedes into the pores of the wick and thus

the menisci at the liquid-vapor interface are highly curved. This

phenomenon is shown in figure. At the condenser end, the menisci at

the liquid-vapor interface are nearly flat during the condensation due

to the difference in the curvature of menisci driving force that


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circulates the fluid against the liquid and vapor pressure losses and

body forces such as gravity.

MechanismHeat pipes use evaporation and condensation to move heat quickly

from one place to another. A typical heat pipe is a sealed tube

containing a liquid and a wick. The wick extends from one end of the

tube to the other and is made of a material that attracts the liquid--the

liquid "wets" the wick. The liquid is called the "working fluid" and is

chosen so that it tends to be a liquid the temperature of the colder end

of the pipe and tends to be a gas at the temperature of the hotter end

of the pipe. Air is removed from the pipe so the only gas it contains isthe gaseous form of the working fluid.

The pipe functions by evaporating the liquid working fluid into gas at

its hotter end and allowing that gaseous working fluid to condense

back into a liquid at its colder end. Since it takes thermal energy to

convert a liquid to a gas, heat is absorbed at the hotter end. And

because a gas gives up thermal energy when it converts from a gas to

a liquid, heat is released at the colder end.

After a brief start-up period, the heat pipe functions smoothly as a

rapid conveyor of heat. The working fluid cycles around the pipe,

evaporating from the wick at the hot end of the pipe, traveling as a gas

to the cold end of the pipe, condensing on the wick, and then traveling

as a liquid to the hot end of the pipe.


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The vapor pressure over the hot liquid working fluid at the hot end of

the pipe is higher than the vapour pressure over fluid at the cooler end

of the pipe (where it condenses), and this pressure difference drives a

rapid mass transfer to the condensing end where the excess vapour

releases its latent heat, warming the cool end of the pipe. Non-condensing gases (caused by contamination for instance) in the

vapour impede the gas flow, and reduce the effectiveness of the heat

pipe, where vapor pressures are low.

The condensed working fluid then flows back to the hot end of the

pipe. In the case of vertically-oriented heat pipes the fluid may be

moved by the force of gravity. In the case of heat pipes containing

wicks, the fluid is returned by capillary action. Most heatpipes used

in heatsinks today have wicks, and are effective in vertical orhorizontal orientation.

In summary: inside a heat pipe, "hot" vapor flows in one direction,

condenses to the liquid phase, which flows back in the other direction

to evaporate again and close the cycle.


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Experimental Procedure

The heat pipe construction is as follows. A copper tube of suitable

length is cleaned thoroughly with suitable cleaning agents. Screen

mesh acts as a wick is wound around a coil in layers and inserted into

the copper tube intact. It is then closed by end caps at both ends.

Thermocouples are equally spaced at various positions of the heat

pipe. The mica sheet is wound over the evaporator region of the heat

pipe since mica is a good electrical insulator and a thermal conductor.

A heating coil is wound over the mica sheet in a uniformly spaced

manner. The two end of the heating coil are connected to the electric

power input. A few centimeter thick cover of glass wool is provided

over the entire region of the heat pipe over the glass wool covering,

the heat pipe is covered with thick PUF insulation which is normally

provided n automobiles.

The heat pipe is evacuated to a pressure of -1.36atm for about 2hours

using a vacuum pump. The heat pipe is tested for holding the vacuum

for about twelve hours. After vacuum test,R-12 working fluid is filled

in the heat pipe for specified pressure which can be indicated by the

pressure gauge.

The coolant water supply is provided to the heat pipe and can be

controlled by a valve. The thermocouples on the heat pipe are

connected to the temperature scanner. A voltmeter is connected in


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parallel to the dimmerstat. Dimmerstat is supplied with ac current.

The temperature scanner is connected to an electric power inlet

through a voltage stabilizer.

The ambient pressure and ambient temperature are noted. The heat

pipe evaporator region heating coil is connected to the electric power

inlet. Coolant water is supplied to the condenser coolant chamber.

The dimmerstat initially is at no-load condition. The load on the

dimmerstat is varied very slowly till the required power is obtained.

Power can be calculated using the equation P=VI cos, where cos

is the power factor, (0.8 for A.C supply).

Heat pipe test rig

The copper tube heat pipe of 25.4 mm inner diameter and thickness

employs a five layered 100x100 brass screen mesh wick. The length

of the evaporator, adiabatic and condenser sections are 100, 50 and

150 mm respectively. The temperature of the heat pipe are measured

using a copper-constantan T-type thermocouples arranged at ten

positions equally spaced along a line on the periphery of the heat pipe.Additionally, two thermocouples are provided to measure the

temperature of coolant inlet and outlet temperatures. The evaporator

region is heated by an electric heating coil wound over a mica sheet.

The condenser region is cooled using coolant flowing through

condenser coolant chamber. Electric power input is varied by using


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dimmerstat. The thermocouples are connected to the 16-channel

temperature scanner.

Experiment

The experimental heat pipe is initially at room temperature. The

coolant water enters the condenser cooling chamber at this

temperature and coolant is allowed to flow at a particular flow rate.

The initial pressure reading is to be noted from the pressure gauge

connected at the evaporator end. The ambient thermocouple

temperatures are noted using thermocouples. Initially, the dimmerstat

is to be kept at no-load condition. The load on the dimmerstat is

slowly varied till it reaches the required value. The electric power is

supplied to the electric heating coil which is wound over the

evaporator section. The temperature at each position on the heat pipe


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can be measured by using the thermocouples connected at equal

intervals. The initial temperature readings are taken in steps of 2

minutes and in later stages the time interval increases to 5 minutes.

After 30-35 minutes the system will reach the steady state conditions.

Study of various parameters

Effect of power

The first experiment is to find out how the temperature profile varies

with respect to the variation provided to the electric power supply

given the electric heating coil in the evaporator region. The

temperature profile with electric power supply of 25 W is plotted for a

time period of nearly one hour till it reaches steady state condition.

Then the electric power varied to 50 Wand 80 W to plot

corresponding temperature profiles. The variation of temperature

profile is then compared.

Effect of pressure

The pressure inside the heat pipe plays an important role in the

temperature profile plotting. The pressure is varied by controlling the

quantity of the working fluid supplied to heat pipe. The plotting of

temperature profile is done on different pressure values. The

combined effect of pressure inside the heat pipe and the power

supplied to electric coil at the evaporator can also be obtained by

varying both parameters


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Effect of coolant supply

The coolant supplied for circulation over the condenser coolant

chamber is water. The coolant flow rate is the ratio of coolant volume

circulating in unit time. The variation in temperature profile is

analyzed at various coolant flow rates.

Effect of working fluid

The effects of various working fluids like R-12,R-22 and R-134a are

analyzed in the experiment. These are commonly used refrigerants.

Limitations

When heated above a certain temperature, all of the working fluid in the heat

pipe will vaporize and the condensation process will cease to occur; in such

conditions, the heat pipe's thermal conductivity is reduced to the heat

conduction properties of its solid metal casing alone. As most heat pipes are

constructed of copper, an overheated heatpipe will generally continue to

conduct heat at only around 1/80th of their original conductivity.


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Conclusion and scope for future work

Our work involves the study of variation on the temperature profile

due to the varied power supply to the evaporator, variation in the

coolant flow rate, variation in the working fluid, variation in the

average pressure inside the heat pipe, variation due to different wick

structures and finally the variation due to geometrical configuration of

the heat pipe. The results of our proposed work provide an insight into

the effect of parameter variation on the temperature profile of a heat

pipe and experiment results will be compared with analyzed results.

Thus we could suggest better ways to improve the performance of a

heat pipe. Further studies on the velocity profile and pressure profile

of a heat pipe can be made and it would be further used for modeling

a heat pipe using any software package such as CFD.


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REFERENCE

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Seminar Heat Pipe

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